Laser gyroscope

ABSTRACT

A laser gyroscope system which detects frequency shifts in which waves are propagated along a path in opposite directions at different frequencies to minimize interaction between the waves with frequency separation of the waves maintained by both reciprocal and nonreciprocable polarization dispersion in which frequencies of the two waves propagating in one direction lie between the two frequencies of waves propagating in the opposite direction. Rotation of the system produces shifts in the frequencies so that variation in the difference between the separation between the two upper frequencies, which propagate in opposite directions, and the separation between the two lower frequencies, which propagate in opposite directions, is used to measure rotation rate.

United States Patent [191 Andringa [m assaaw Dec. 17, 1974 LASERGYROSCOPE [76] Inventor: Keimpe Andringa, 8 Course Brook Rd., Sherborn,Mass. 01770 [22] Filed: Oct. 2, 1972 21 App]. No.: 294,394

Related US. Application Data Primary Examiner-Ronald L. Wibert AssistantExaminer-V. P. McGraw Attorney, Agent, or Firm-Joseph'D. Pannone; MiltonD. Bartlett; David M. Warren 57 ABSTRACT A laser gyroscope system whichdetects frequency shifts in which waves are propagated along a path inopposite directions at different frequencies to minimize interactionbetween the waves with frequency separation of the waves maintained byboth reciprocal and nonreciprocable polarization dispersion in whichfrequencies of the two waves propagating in one direction lie betweenthe two frequencies of waves propagating in the opposite direction.Rotation of the system produces shifts in the frequencies so thatvariation in the difference between the separation between the two upperfrequencies, which propagate in opposite directions, and the separationbetween the two lower frequencies, which propagate in oppositedirections, is used to measure rotation rate.

19 Claims, 3 Drawing Figures i "56 l '1" DIFFERENTIAL I AM LIFIER I I lCOUNTER Q L- -J 7 COUNTER PATENIEU LEE] H974 sumawg 230 mw OL FREQUENCYLASER GYROSCOPE RELATED lNVENTlONS This is a continuation-in-part ofapplication Ser. No. 120,581, filed by Keimpe Andringa on Mar. 3, 1971now US. Pat. No. 3,741 ,657 and assigned to the same assignee as thisinvention.

BACKGROUND OF THE INVENTION Laser gyroscopes have been proposed in whichwaves travel in opposite directions through a laser medium so thatrotation of the laser medium about an axis will produce a differencefrequency. However, unless the frequencies of the waves are spaced asubstantial distance apart, the coupling of the wave' propagating in onedirection with the wave propagating in the opposite direction in thelaser material can produce a combined laser action pulling the twofrequencies toward each other and producing a condition known aslock-in.

Lock-in limits the use of'laser gyroscopes since at low rotation rateswhere lock-in normally occurs, the output drops to zero, producing arange of rotation rates where there is no output since the clockwise andcounterclockwise waves have the same frequency. Coupling between thewaves may occur in many ways, including back scattering of energy fromelements of the laser system such as window interfaces or othertransitions from one medium into another.

Another possible source of coupling within the laser medium itselfoccurs when two waves travelling in opposite directions are in aninstantaneous phase relationship where they compete for gain from atomshaving a low velocity in the direction of propagation of the waves. Theprobability of lock-in which affects the width of the inaccurate zerooutput region of the gyro in general will increase as the loop gain ofthe laser increases.

If the frequencies are spaced apart a substantial distance, for exampleby the use of devices thatproduce different delays in one direction thanin the other, this frequency difference must be accurately maintained.Attempts to achieve an accurate frequency separation by switching aFaraday rotator from one condition to another have proved impracticalsince the accuracy of the AC switching waveform must be perfectlysymmetrical to a degree substantially greater than one part in Inaddition, if a non-switched Faraday rotator were used to produce thedifferent frequencies for opposite propagation directions, variations inthe Faraday rotator could produce frequency variations greater than thegyroscopic rotational frequency changes, hence rendering the systeminaccurate.

SUMMARY OF THE INVENTION In accordance with this invention, two pairs ofwaves propagating in opposite directions are maintained spaced infrequency to substantially reduce coupling toeach other, and frequencyshifts due to changes in operating temperature, supply energy levels ormechanical'movement of elements of the system with respect to each otherwill substantially cancel so that the overall output of the system willbe unaffected by such changes.

More specifically, this invention provides for a ring laser having atleast a plurality of waves propagating in each direction around thelaser ring path. A plurality of different oscillation frequencies arepropagated in each direction around the ring path. The propagation timesof the waves are such that the frequencies of a pair of waves travellingin one direction through the laser lie between the frequencies of a pairof waves travelling in the opposite direction through the laser.Movement of the laser ring path, for example by rotation of the systemabout an axis perpendicular to the path, producesfrequency shifts of thepair of waves propagating in one direction through the laser which areopposite to the frequency shifts of waves moving in the opposite direc-I Since frequency shifts due to variations in power supply input,mechanical vibrations of components, or thermal variations in the systemwill shift all frequencies substantially equally because all the wavestravel through the same components, the separations of the higher andlower frequencies of each pair will deviate in the same direction, henceresulting in a zero total deviation. Therefore, such frequency shiftsintroduce no substantial errors into the system.

This invention also provides for operating the system such that thelower frequency of each pair is positioned below the maximum gainfrequency of the laser transition resonance energy band, while the upperfrequency of each pair is positioned above said maximum gain frequency.More specifically, the frequencies of each pair are maintainedsubstantially equidistant above and below the maximum gain or centerfrequency of the laser by any desired means, such as varying the pathlength of all the waves as a function of the difference between theamplitude of 'the two lower frequencies and the two upper frequencies.In general, the laser has a substantially Gaussian distribution offrequency versus gain, and any tendency of the two upper and/or twolower frequencies to pull'together due to the slope of. the gain curve,or variations therein, will cause the separation between the two lowerfrequencies and the separation between the two upper frequencies todeviate in the same direction. Therefore, the total deviation in thesefrequency separations is substantially zero for medium by making use ofdifferent-polarizations of waves. More specifically, a pair ofcircularly polarized waves of opposite sense travel in each directionthrough the medium. By the selection of a medium such as a quartzcrystal oriented for the waves to travel along the optical axis, thedelay time for a wave of one polarization will be different from thedelay time of a wave of a different polarization. The difference indelay time, which is reciprocal and which results in the differentfrequencies, may be selected by selecting the lengt of the quartzcrystal.

In addition, the small Faraday effect, which many crystals will exhibit,is used to produce a non-reciprocal change in the delay time forcircularly polarized waves travelling in opposite directions through thecrystal so that the delay time is different for a wave of onepolarization passing through the crystal in one direction from the delaytime of a wave of the same polarization passing through the crystal inthe opposite direction.

The amount of Faraday rotation produced varies with the strength of themagnetic field applied axially parallel to the propagation directionthrough the crystal, and the direction of the Faraday rotation can bereversed by reversing the direction of the magnetic field. Variations inthe magnetic field will cause shifts in all four frequencies of thesystem, with shift in the frequencies of one sense of polarizationmoving in an opposite direction since the waves of that polarizationsense of rotation are also travelling in opposite directions through theFaraday rotator. As a result, variations in the magnetic field willcause separation between the upper frequency of each pair and separationbetween the lower frequency of each pair to deviate in the samedirection so that the total deviation is zero. Accordingly, ,thesefrequency shifts do not substantially affect the output of thegyroscope.

DESCRIPTION OF THE DRAWINGS Other and further objects and advantages ofthe invention will become apparent as the description thereofprogresses, reference being had to the accompanying drawings wherein:

. FIG. 1 illustrates a diagrammatic view of a ring laser I embodying theinvention;

FIG. 2 illustrates a diagram of operating characteristics of the systemillustrated in FIG. 1; and

tem.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 illustrates a preferredembodiment of the sy s- I of the laser 10, and defining the rectangularpath with the laser amplifier 10 in one leg of the path.

Positioned in the opposite leg of the rectangle from the laser 10 is apolarization dispersive structure 16 which delays waves of differentpolarizations by different amounts. Structure 16 may be, for example, aconventional quartz crystal rotator 17 having its optical axis parallelto the laser path, and a conventional Fara day rotator 18 having itsmagnetic field parallel to the path of the laser wave.

Crystal rotator 17 produces a delay for circularly polarized wavesthatis different for one sense of circular polarization than for theopposite sense and is reciprocal, that is, a wave travelling in eitherdirection through the crystal will be delayed the same amountFaradayrotator 18 also delays circularly polarized waves but is non-reciprocal,that is, it rotates a circularly polarized wave of one sense propagatingthrough the rotator in one direction in a positive sense, or increasesthe amount of rotation, but rotates the same sense of circularlypolarized wave propagating through the rotator in the same direction ina negative sense, or decreases the amount of rotation.

Since change in the total amount of delay changes the electrical pathlength, and since an integral number of wavelengths around the laserbeam path, defined by elements 10, ll, l2, 13, 1'4, l5, l7 and 18, isrequired to produce oscillations, four frequencies of oscillation areproduced. The frequencies in FIG. 2 corresponding to 21 and 22 may be,for example, produced by left hand circularly polarized waves with thefrequency 21 travelling in a clockwise direction about the laser systemillustrated in FIG. 1 and the frequency 22 travelling in acounterclockwise direction about the system of FIG. 1. Frequenciescorresponding to 23 and 24 represent right hand circular polarizationwaves with frequency 23 travelling in a counterclockwise direction andfrequency 24 travelling in a clockwise direction aroundthe laser systemillustrated in FIG. 1. These frequencies are shown as positive ornegative differences from the center or maximum gain frequency of thelaser 10.

When the system of FIG. 1 is rotated about an axis perpendicular to theplane of the laser path, frequencies 22 and 23 will both shift in onedirection, while frequencies 21 and 24 will both shift in the oppositedirection. For example, if the system is rotated in the clockwisedirection, frequencies 22 and 23 will be reduced and frequencies 21 and24 will be increased, and since frequencies 22 and 23 lie betweenfrequencies 21 and 24, the separation between frequencies 21 and 22 willbe reduced and the separation between frequencies 23 and 24 will beincreased. The sum of the changes in said separations is directlyproportional to the rate of rotation of the system of FIG. 1. This sumis separated from frequency shifts due to other causes, such as gainvariation, or thermal changes in path length, by algebraically addingsaid changes, or deviations in said frequency separations. A decreasein's uch a frequency separation will have a negative sign and anincrease in such a frequency separation will have a positive sign.

The direction of rotation is determined by the relative signs of saidfrequency deviations. For example, when the separation betweenfrequencies 21 and'22 is less than the separation between frequencies 23and 24, the system is rotating clockwise, and when the separationbetween frequencies 21 and 22 is greater than the separation betweenfrequencies 23 and 24, the rotation is counterclockwise.

The four frequencies of the gyroscope are obtained .from the smallamount of laser wave energy which will pass through the mirror 14 insmall quantities, for example less than one-tenth'of one percent of thetotal energy of the beam. Clockwise propagating waves I propagatethroughmirror 14 along the beam path shown at 30 whereascounterclockwise waves propagate along the beam path 31. The beams passthrough quarter wave plates 32 of any desired type, such as, for

ized wave produced by the left hand circular polarization. The beams 30and 31 are then each divided into two substantially equal amplitudebeams by half silvered mirrors 33, the beams passing through halfsilvered mirrors 33 being reflected by mirrors 34 to produce four beamswhich are passed through four polarization analyzers 35. Thepolarization analyzers pass only one angle of linearly polarized waveand, by adjusting the rotational orientation of the analyzers 35,produce beams 41, 42, 43 and 44 having substantially only thefrequencies 21, 22, 23 and 24, respectively.

Beams 41 and 42 are superimposed by means of a half silvered mirror 45on a photodiode 46, and beams 42 and 43 are superimposed by means of ahalf silvered mirror 47 on a photodiode 48. Photodiodes 46 and 48 areback biased by means of batteries 49 and 50, respectively, and thedifference frequency produced by photodiode 46 from the separationbetween frequencies 21 and 22 appears across load resistor 51, while thedifference frequency produced by photodiode 48 from the separationbetween frequencies 23 and 24 appears across load resistor 52. Thefrequencies above these difference frequencies are filtered by the straycapacitance of the system and do not appear across resistors 51 and 52.

The difference frequencies are counted by counters 53 and 54, which mayinclude forming or shaping circuits to generate digital pulses from thesinusoidal difference frequency waves in accordance with wellknownpractice. The output of counter 53 is subtracted from the output ofcounter 54 by means of an adder 55 which is connected to add the outputof counter 53 as a negative number to the output of counter 54as apositive number, in accordance with well-known counter practice. Theoutput of the adder 55 is, therefore, a number directly proportional tothe total amount of rotation of the laser system over the period of timeduring which the counters have counted. 1f the total count is positive,the output of counter 54 is greater than the output of counter 53 andthe rotation is in the 'counterclockwise direction whereas, if theoutput is negative, the rotation is in the clockwise direction. Inaccordance with well-known computer practice, the counters may be madeto repetitively count for predetermined periods and the count is thenproportional to rotation rate.

The signals developed across resistors 51 and 52 will have an amplitudedependent on the position of the frequencies on the gain curve 90illustrated in FIG. 2 and may be used to produce a control signal tocenter frequencies 21, 22, 23 and 24 symmetrically about the centerfrequency of curve 90. The amplitudes of these signals are detected bydiodes 61 and 62, respectively, and appear across load resistors 63 and64, respectively, whose outputs are sent through filter networkscomprising resistors 65 and condensers 66, which determine the desiredfrequency response of the control signal loop to a differentialamplifier 67, whose output drives a piezoelectric crystal supporting oneof the mirrors 12. As shown here, the piezoelectric crystal is a blockof quartz 68, having a back electrode 69 which may be part of themechanical support (not shown), and a front electrode 70 supporting themirror 12. The amplification and polarity of the differential amplifier67 are selected to produce movement of the mirror 12 to compensate formechanical movement of portions of about the center maximum gainfrequency of the curve 90.

Referring now to FIG. 3, there is shown a preferred embodiment of theinvention in which the laser 10 is a gas laser having a glass envelope71 containing a mixture of neon and helium which will amplifywavelengths of approximately 6,328 Angstroms. Two cathodes 72 arepositioned in side arms 73 adjacent the ends of the laser 10 which areconnected to the laser bore 74extending between the ends of the glassbody 71 and terminating at optical windows 75. An anode 76 is positionedin a side arm 77 connected to the laser bore 70 approximately midwaybetween the cathode arms 73. Direct current electrical discharges-areproduced between the cathodes 72 and the anode 76 by means of a powersupply 78 which is preferably of the adjustable v constant current type.

The laser path is defined by four mirrors 80, 81, 82 and 83 positionedat the corners of a substantially rectangular path at substantially 45with respect to the angles of incidence and reflection of the path. Theentire assembly defining the path is mounted on a support 84 whichprovides a substantially rigid positioning of the elements to minimizefluctuation of the path length due to mechanical vibration. Support 84is mounted on a system (not shown) whose rotation is to be measured.Mirrors 80, 81, 82 and 83 are made adjustable in accordance with wellknown practice by mounts (not shown) in order to line up the laser beamalong the rectangular path. One or more of the mirrors, such as mirror82, is made concave to aid in concentrating the beam through thelaser'bore 74. The entire assembly may, if desired, be protected fromerrors due to the movement of gas along the laser path by evacuating allportions of the laser path other than those inside the gas filledenvelope 71. Movement of the gas in the laser bore 74 due to theelectrical discharge is compensated for since ion motion in the gasalong the laser bore 74 is in both directions in the bore 74 from anodearm 77 toward each of the cathode arms 73. Therefore, when the laser isexcited by a direct current discharge, equal and opposite'motion of thegas particles occurs within the laser bore 74. The length of the bore 44is made sufficient to overcome the losses in the laser signal along thereentrant path and may be, for example, 20 to 100 centimeters long. Thetotal path length may be shortthe system with respect to each other,thereby keeping frequencies 21, 22, 23 and 24 positioned symmetricallyened by positioning additional lasers in the other legs of therectangular path, and if desired, one or more mirrors may be made asportions of the windows to reduce the loss. I

In order to insure that the laser does not produce waves at frequenciesother than in the desired molecular resonance band, the reentrantpath ispreferably made frequency responsive such that it has a lower loss atthe desired resonance band of frequencies. Such a frequency responsivefilter characteristic may be achieved by making one or more of themirrors'l 2, l3,

different from the desired frequency band, the waves pass through thecoating and the glass, hence making the reentrant path highly lossy atsuch other frequencies.

In the leg opposite to that containing the laser 10, there is. shown apolarization dispersive structure 85 comprising a body of quartz crystalpositioned with its Z, or optical axis parallel to the path of the laserbeam.

This produces the reciprocal polarization dispersion produced by theelement 17 in FIG. 1.

If the frequency separation produced by reciprocal polarizationdispersion is relatively large, for example 150 megacycles, Faradayeffect in the quartz crystal will produce a sufficient non-reciprocalpolarization dispersion to produce a frequency separation of, forexample, 0.1 percent of that produced by the reciprocal polarizationdispersion. The Faraday rotation is produced by a permanent magnet 86positioned between magnetic pole pieces 87 and 88 at the ends of thecrystal 85 which produces a magnetic field axial to the laser beam. Polepieces 87 and 88 have apertures therein to permit passage of the laserbeam. Since variations in the magnetic field produce frequency shiftswhich cancel in output signal, the size of the magnetic field is notcritical and is selected for the length of crystal 85 to produce asufficient non-reciprocal polarization shift in accordance with theknown physical constants of quartz. Any materials which have the desiredreciprocal polarization dispersion and non-reciprocal Faradaypolarization dispersion can be used in place of quartz. For the presentembodiment a quartz crystal length of approximately 4 millimeters and amagnetic field strength of 2,000 gauss will produce the desiredfrequency separations.

Light coupled out of the system, for example, as small amountstransmitted through mirror 83, will impinge on an output structure 89comprising quarter wave plates, half silvered mirrors, polarizationanalyzers and photodetectors of any desired configuration such as, forexample, that illustrated in FIG. 1. For the embodiment illustrated inFIG. 3 the laser will have a relatively sharp amplification curve due tothe molecular resonance illustrated as curve 90 in FIG. 2 in which thehalf power points are spaced about 1,000 megacycles apart as illustratedby the points 91. In such a system having relatively long laserpath'length, adjacent modes of oscillation can appear above the halfpower points. These modes are, for the configuration shown, between 300and 400 megacycles away from the desired operating frequencies 21, 22,23 and 24, as illustrated at frequencies 92. Since the curve 90 isessentially a Gaussian distribution of gain versus frequency, the gainof the system may be adjusted by adjusting supply 70 so that the loopgain is less than unity for frequencies in those regions of curve 90 inwhich the adjacent modes 92 lie, for example, as shown by the portion ofcurve 90 below line 93. The modes 92 will then not be excited, and anydifferences in frequency which those modes might introduce will beeliminated.

If desired, the adjacent modes may be excited and under these conditionsthe system may be operated with the frequency separation due tonon-reciprocal Faraday rotation not exceeding a few hundred kilohertz.The output photodetectors, for example 46 and 48 in FIG. 1, can then bedesigned along with the load resistors 51 and 52 to have shuntcapacitance which filters out all frequencies above I megahertz or soand,

v tures and operating currents, and solid lasers using, for

modes. These small bands can be. averaged in the computer counters'53and 54 or in a frequency discriminator circuit in accordance withwell-known practice. In this mode of operation, stabilization of thepath length, for example by a quartz crystal on a mirror, can, ifdesired, be eliminated since adjacent modes will always be within thegain curve.

It should be clearly understood that the details of the foregoingembodiments are set forth by way of example only, and any type of lasercould be used. The width of the gain curve can be adjusted by adjustinggas mixexample, ruby, neodymium doped yttrium aluminate garnet, orneodymium doped yttrium ortho-aluminate may be used. While, in general,the accuracy of the system for a given size increases with the frequencyregion, it is contemplated that the principles of this invention areequally applicable at lower frequencies, such as for example themicrowave region, and that amplifiers such as semiconductor devices canbe used in place of the distributed laser amplifier illustrated herein.Accordingly, it is contemplated that this invention be not limited bythe particular details of the embodiments illustrated herein except asdefined by the appended I claims.

What is claimed is: 1. In combination: means for simultaneouslydirecting a plurality of radiant energy waves having at least-aplurality of substantially coherent frequencies in opposite directionsthrough a polarization dispersive medium, means comprising a mediumcommon to at least a portion of the path of each of said waves foramplifying said waves; frequency responsive reflecting filter means insaid path; and means for varying said frequencies comprising means formoving said medium. 2. The combination in accordance with claim 1wherein said path is defined at least in part by a plurality of saidfrequency responsive reflecting filter means.

3. The combination in accordance with claim 2' wherein a plurality ofsaid waves shift in frequency in opposite directions as a function ofthe rotation of said path around an axis. V

4. The combination in accordance with claim 3 wherein said amplifyingmeans comprises means for producing a population inversion in at leastone of a plurality of possible energy states insaid medium.

5. The combination in accordance with claim 4 wherein said mediumcomprises a fluid.

6. The combination in accordance with claim 5 wherein said mediumcomprises a mixture of helium and neon.

7. The combination in accordance with claim 1 wherein the free spacewavelengths of said waves are substantially 6,328 Angstroms.

8. The combination in accordance with claim 1 wherein polarizationdispersive means are positioned within said structure.

9. The combination in accordance with claim 8 wherein said polarizationdispersive means provide both reciprocal polarization dispersion andnonreciprocal polarization dispersion of said waves.

10. In combination:

means for simultaneously directing waves having at least a plurality ofsubstantially coherent frequencies in opposite directions along a path;

frequency responsive reflecting filter means;

means in said path comprising a medium common to all of said waves foramplifying said waves; and

means for varying the length of said path as a function of thevariations in frequencies of all of said 14. The combination inaccordance with claim 13 wherein at least two of said waves travellingin the same direction along said path have substantially differentpolarizations.

15. The combination in accordance with claim 14 wherein said amplifyingmeans comprises a medium extending along at least a substantial portionof said path.

16. The combination in accordance with claim 15 wherein said medium hasa plurality of possible energy states.

17. The combination in accordance with claim 16 wherein said amplifyingmeans comprises means for producing a population inversion in at leastone of said energy states.

18. The combination in accordance with claim 17 wherein said mediumcomprises a fluid.

19. The combination in accordance with claim 18 wherein said gascomprises helium and neon.

1. In combination: means for simultaneously directing a plurality ofradiant energy waves having at least a plurality of substantiallycoherent frequencies in opposite directions through a polarizationdispersive medium; means comprising a medium common to at least aportion of the path of each of said waves for amplifying said waves;frequency responsive reflecting filter means in said path; and means forvarying said frequencies comprising means for moving said medium.
 2. Thecombination in accordance with claim 1 wherein said path is defined atleast in part by a plurality of said frequency responsive reflectingfilter means.
 3. The combination in accordance with claim 2 wherein aplurality of said waves shift in frequency in opposite directions as afunction of the rotation of said path around an axis.
 4. The combinationin accordance with claim 3 wherein said amplifying means comprises meansfor producing a population inversion in at least one of a plurality ofpossible energy states in said medium.
 5. The combination in accordancewith claim 4 wherein said medium comprises a fluid.
 6. The combinationin accordance with claim 5 wherein said medium comprises a mixture ofhelium and neon.
 7. The combination in accordance with claim 1 whereinthe free space wavelengths of said waves are substantially 6,328Angstroms.
 8. The combination in accordance with claim 1 whereinpolarization dispersive means are positioned within said strUcture. 9.The combination in accordance with claim 8 wherein said polarizationdispersive means provide both reciprocal polarization dispersion andnonreciprocal polarization dispersion of said waves.
 10. In combination:means for simultaneously directing waves having at least a plurality ofsubstantially coherent frequencies in opposite directions along a path;frequency responsive reflecting filter means; means in said pathcomprising a medium common to all of said waves for amplifying saidwaves; and means for varying the length of said path as a function ofthe variations in frequencies of all of said waves.
 11. The combinationin accordance with claim 10 wherein at least two pairs of said wavestravel in opposite directions through said medium.
 12. The combinationin accordance with claim 11 wherein frequency differences between saidwaves vary as a function of movement of said path.
 13. The combinationin accordance with claim 12 wherein said path is defined at least inpart by a plurality of said frequency responsive reflecting filter meanspositioned respectively at a plurality of spaced locations along saidpath.
 14. The combination in accordance with claim 13 wherein at leasttwo of said waves travelling in the same direction along said path havesubstantially different polarizations.
 15. The combination in accordancewith claim 14 wherein said amplifying means comprises a medium extendingalong at least a substantial portion of said path.
 16. The combinationin accordance with claim 15 wherein said medium has a plurality ofpossible energy states.
 17. The combination in accordance with claim 16wherein said amplifying means comprises means for producing a populationinversion in at least one of said energy states.
 18. The combination inaccordance with claim 17 wherein said medium comprises a fluid.
 19. Thecombination in accordance with claim 18 wherein said gas compriseshelium and neon.